This proposal seeks to determine the mechanisms and receptors by which leukotriene (LT)E4, the most stable ligand of the cysteinyl leukotrienes (cys-LTs), activates mast cells (MCs) and promotes pulmonary inflammation. An abundance of historical information supports the existence of a distinct receptor for LTE4 Our published studies demonstrate that LTE4 is the most potent cys-LT for both for human MC proliferation in vitro and potentiation of airway inflammation in vivo, and it activates human MCs by a mechanism independent of CysLT1R and CysLT2R. Our Preliminary Studies strongly indicate that the P2Y12 receptor, an adenosine diphosphate-responsive GPCR, is also the LTE4-reactive receptor (hereafter termed P2Y12/CysLT3R) mediating the effects of LTE4 on MCs in vitro and on lung pathology in vivo. The central hypothesis is that LTE4, the weakest agonist of the cys-LTs at their known receptors, is the most relevant cys-LT for promoting allergic mucosal inflammation. A corollary hypothesis is that the actions of LTE4 are mediated by a previously unrecognized cys-LT-reactive G protein-coupled receptor (GPCR), hereafter referred to as "P2Y12/CysLT3R". Aim 1 uses in vitro approaches to prove that P2Y12/CysLT3R is a true LTE4-reactive receptor. Aim 2 seeks to prove that P2Y12/CysLT3R accounts for the striking proinflammatory effects of LTE4 in the lung in vivo, and to identify the essential cellular targets of this effect. The findings are expected to have immediate implications for asthma pathophysiology and treatment.